1. Field of the Invention
The present invention relates to a mechanical safety breaker to be incorporated in a mechanical connection to sever the connection when a force or torque transmitted therethrough increases beyond a predetermined value.
2. Prior Art
The mechanical safety breaker adapted to operate normally to transmit a force or torque therethrough but to get broken when the magnitude of the force or torque increases beyond a predetermined limit value so as thereby to cease the transmission of the force of torque for the safety purpose is known and used in various mechanical devices. Conventionally, a breaker element which forms an essential part of a force or torque transmitting construction of such a mechanical safety breaker and is fractured by an overloading is generally made of metal or resin generally known as having high impact resistance. However, there is a problem in making the breaker element by metal or resin, because the fracture strength of metal or resin generally lowers much due to fatigue when it has been subjected to repetitive loadings. Therefore, when the breaker element of the mechanical safety breaker is made of metal or resin, there occurs a great inconvenience that the breaker is undesirably actuated after a period of use even when the mechanical load to be transmitted is still below a limit value predetermined for its actuation.
In Japanese Patent Laid-open Publication 61-220968 it is shown to incorporate a mechanical safety breaker in an electric motor type power steering system of a vehicle such as an automobile such that an electric motor is drivingly connected with the steering shaft via a breaker element (sheer pin) adapted to be broken when the sheering force applied thereto increases beyond a predetermined value, considering that, in the electric motor type power steering system which generally includes an electronic control system and controls supply of electric power to the electric motor to be generally increased in accordance with increase of the manual force for operating the steering wheel, the electronic control system is not so free from any troubles as the simply mechanical steering shaft system, and further, since a reduction gear train is generally incorporated between the electric motor and the steering shaft driven thereby, if the supply of the electric power to the motor fails or improperly lowers, a high resistance is applied to the manual operation of the steering shaft. However, such a breaker element is subjected to highly frequently repetitive applications of alternating load due to the steering operation of the vehicle, rendering the fracture strength thereof to quickly fall due to fatigue. Nevertheless, it is highly required that the limit value of the steering torque at which the mechanical safety breaker of the power steering system is actuated is stably maintained within a relatively narrow range for a long time such that the mechanical safety breaker should not respond to such a temporal increase of the steering torque as will be caused by a bumping on of a steering vehicle wheel on a curbstone or the like, while ensuring that the steering shaft is released from the electric motor for free manual operation of the steering any time when a trouble has occurred in the assisting power steering system.